Cellulose is a β-1,4-linked polymer of anhydroglucose. Cellulose is typically a high molecular weight, polydisperse polymer that is insoluble in water and virtually all common organic solvents. The use of unmodified cellulose in wood or cotton products such as housing or fabric is well known. Unmodified cellulose is also utilized in a variety of other applications usually as a film, such as cellophane, as a fiber, such as viscose rayon, or as a powder, such as microcrystalline cellulose used in pharmaceutical applications. Modified cellulose such as cellulose esters are also widely utilized in a wide variety of commercial applications [Prog. Polym. Sci. 2001, 26, 1605-1688]. Cellulose esters are generally prepared by first converting cellulose to a cellulose triester before hydrolyzing the cellulose triester in an acidic aqueous media to the desired degree of substitution (DS, the number of substituents per anhydroglucose monomer). Hydrolysis of cellulose triacetate under these conditions yields a random copolymer that can consist of 8 different monomers depending upon the final DS [Macromolecules 1991, 24, 3050].
The dissolution of cellulose in ionic liquids is known. Those skilled in the art will recognize that the maximum amount of cellulose dissolved at any given temperature in a particular ionic liquid will depend upon the degree of polymerization (DP) of the initial cellulose and the extent of degradation of the cellulose during the dissolution process.
In the broadest sense, an ionic liquid (IL) is simply any liquid containing only ions. Hence, molten salts such as NaCl which melts at temperatures greater than 800° C. could be classified as ionic liquids. From a practical point of view, the term ionic liquid is now used for organic salts that melt below approximately 100° C.
Although the cations of ionic liquids are structurally diverse, they generally contain nitrogen or phosphorus that can be converted to a quaternary ammonium or phosphonium. In general, the most useful ionic liquids contain nitrogen that is part of a ring structure. Examples of these cations include pyridinum, pyridazinium, pyrimidinium, pyrazinium, imidazolium, pyrazolium, oxazolium, triazolium, thiazolium, piperidinium, pyrrolidinium, quinolinium, and isoquinolinium.

The anions of ionic liquids can also be structurally diverse. The anion can be either inorganic or organic and they can have a significant impact on the solubility of the ionic liquids in different media. For example, ionic liquids containing hydrophobic anions such as hexafluorophosphates or triflimides have very low solubilities in water while ionic liquids containing hydrophilic anions such chloride or acetate are completely miscible in water.
The names of ionic liquids are generally abbreviated. Alkyl cations are often named by the letters of the alkyl substituents and the cation which are given within a set of brackets followed by the abbreviation for the anion. Although not expressively written, it is understood that the cation has a positive charge and the anion has a negative charge. For example, [BMIm]OAc means 1-butyl-3-methylimidazolium acetate and [AMIm]Cl means 1-allyl-3-methylimidazolium chloride.
In the case in which nitrogen is part of a cyclic cation (e.g. imidazolium), these ionic liquids typically have lower melting points, are less hydroscopic, and are more stable that the corresponding ammonium or phosphonium containing ionic liquids. In terms of cellulose dissolution and esterification, ionic liquids containing imidazolium as the cation are generally preferred. With these ionic liquids, it is possible to achieve concentrated cellulose solutions (ca. 20 wt % in [EMIm]OAc) from which a variety of cellulose esters can be prepared (U.S. Pat. No. 6,824,599, US application 20080194807, US application 20080194808, and US application 20080194834).
The most commonly utilized non-cyclic ammonium based ionic liquids are tetraalkylammonium halides. This class of ionic liquids has limited utility in cellulose dissolution and esterification. Only two tetraalkylammonium halides, tetraethylammonium chloride (U.S. Pat. No. 4,597,798) and tetrabutylammonium fluoride hydrate (Macromol. Biosci. 2007, 7, 307-314), have been shown to solubilize cellulose at a significant level (ca. 6 wt % cellulose). In both cases, significant concentrations of cosolvents (30-90 wt %) such as DMSO or DMF were required for cellulose solubilization. Due to low concentrations of cellulose in the ionic liquids, the difficulty of removing water from theses ionic liquids, the instability of these ionic liquids, as well as the corrosive and toxic nature of these ionic liquids, these tetraalkylammonium halides are not practical media for making cellulose derivatives.